William F. Weld, the Libertarian Party nominee for vice president, defended Hillary Clinton on Wednesday against attacks from Donald J. Trump, insisting her character should not be impugned by someone as flawed as he is.

Mr. Weld made the remarks in a brief interview with The New York Times, the latest comments he has made as part of a highly unusual move to use his remaining time in the race standing up for one of his rivals instead of making the case for his own running mate, Gary Johnson.

“I’m sick and tired of seeing her dragged through the mud by a man who makes absolutely no demands on himself as to moral behavior, including common decency or truthfulness,” Mr. Weld said.

His comments come the morning after an interview with Rachel Maddow on MSNBC, in which he discussed his decision to “vouch” for Mrs. Clinton. “I think it’s high time somebody did,” he said.

Mr. Weld, a former Republican governor of Massachusetts, told The Boston Globe in early October that he was shifting his approach to the race, speaking out in support of Mrs. Clinton and against Mr. Trump, whom he has described as running a campaign in a tradition of fascism.